In programming the output of a direct current power supply a digital to analogue converter may convert the programmed digital information representing volts or amperes provided by the computer into a corresponding preliminary analogue voltage control signal. From the signal an analogue reference voltage control signal is derived which can be used as the reference voltage of the power supply. As the reference voltage changes, the output of the power supply changes in a corresponding manner.
Because it is derived from digital information, the output signal of the power supply can be accurate at a number of points determined by digital information. Therefore, if the output voltage of the supply should be limited to a range of 10 volts, the accuracy at any point will be greater than it would be if the range extended to 50 volts. If both ranges of operation are required, the program for the computer includes information as to the range, and the converter provides a range control signal. The signal is used to control a switch that changes the amplitude of the reference voltage control signal by simply changing the gain of amplifiers that provide it.
It has been found that a system as just described produces transient increases in the output voltage of the power supply when ranges are changed that can severly damage equipment connected to its output.